The present invention relates to a method and apparatus for separating artificial drill cuttings introduced into a drilling fluid, ("mud"), which is pumped into a drilling string in a borehole, from a mixture of artificial and natural drill cuttings contained in drilling fluid returning to the surface from the borehole.
In recent years the pressure for production of increased oil and gas has led to increased use of wells drilled at angles with respect to the vertical, sometimes called deviated wells. This is done for a variety of reasons, for example, to effect a wide area of coverage from a single well head, such as an off-shore oil platform. Other reasons for drilling at angles include avoiding drilling through large upthrusts of material located directly over formations of interest, and avoiding having to drill through corrosive material, such as found in salt domes. The art recognizes that there are other applications of drilling at angles to the vertical as well.
The typical well drilling procedure, particularly in exploration for oil, gas and other minerals involves the rotation of a drill bit at the end of a long tubular drill string. A drilling fluid or "mud" is pumped down the center of the drill string, to exit around the hollow drill bit, and carries the cuttings drilled from the subsurface formation back to the surface for removal. The mud also provides a cooling effect to the drill head. In a well drilled along the vertical the cuttings are carried to the surface by the mud passing around the drill head on all sides. However, in a well drilled at an angle to the vertical, the tendency is for the mud to flow upwardly along the upper side of the drill string because it is typically of lower density than the metallic drill string. The absence of a mud current on the lower side of the drill string allows a bed of cuttings to build up under the drill string. This can be a severe problem if the cuttings bed builds up to such an extent that the drill string is partially or fully embedded in the cuttings bed. When one tries to remove the drill head, which is typically of larger diameter than the drill string, the cuttings bed prevents or impedes its removal.
Numerous expedients have been suggested for ensuring that cuttings are removed properly from a well drilled at an angle to the vertical and some of these are successful; however, one area which the art has not addressed is the provision of a method for determining whether the cuttings are, in fact being properly removed from a well, so as to determine whether any of these methods should be used, and, if they are used, whether or not they are working effectively.
The above-referenced U.S. application discloses and claims a method of monitoring the growth of a cuttings bed in an angled well. The method employs the step of injecting artificial drill cuttings, which have a density comparable to natural drill cuttings, but which are distinguishable, e.g. magnetically, from natural drill cuttings, into a drilling fluid, e.g., mud, pumped down the drill string during well drilling, and monitoring the rate of return of the artificial cuttings with respect to the rate of injection to determine the efficacy of the removal of natural cuttings generated during drilling. By practicing this method, one can obtain an indication on whether a cuttings bed is increasing or decreasing in size or is substantially stable in size, and an indication of whether natural drill cuttings are being properly removed from a borehole during drilling.
In practicing the above method, it is desirable to recycle and reuse the artificial drill cuttings for reasons of economy. Accordingly, one object of the present invention is the provision of a method and apparatus for economically and reliably separating the artificial drill cuttings for reuse from a mixture containing both artificial and natural drill cuttings contained in a returning drilling fluid.
Another object of the invention is the provision of a method and apparatus for economically and reliably determining a rate and/or total amount of artificial drill cuttings which are separated from a mixture containing both artificial and natural drill cuttings contained in a returning drilling fluid.
Another object of the invention is the provision of a method and apparatus for economically and reliably separating materials having magnetic properties from a mixture of those materials and others which do not have magnetic properties.